N. meningitidis LOS mimic human glycolipids in having lacto-N-neotetraose ( LNnt, Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc ) sequence in the oligosaccharides. The oligosaccharides of N. meningitidis LOS have been reported to have a triatennary structure with Lnnt at the nonreducing end of the longest antenna in the branched oligosaccharides. We have used a mouse monoclonal antibody ( anti-My-28 ) which recognized Lnnt, to investigate this sequence in the oligosaccharides of the LOS. Eight of the twelve immunotype LOS, all but types 1, 6, 11, and 12, bound the antibody as measured by ELISA, immunodot and immunoblot assays. N-Acetyllactosamine inhibited the binding of the antibody to all eight reactive LOS. The antibody binding to a representative LOS ( type 2 ) was best inhibited by Lnnt, next by N- acetyllactosamine, but not inhibited by galactose, Galbeta1-3GlcNAc, and lacto-N-tetraose, Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc. These results suggest that the Lnnt sequence is present in 8 of 12 immunotype LOS. The presence of the Lnnt sequence in the LOS, which is found in paragloboside (Lnnt-ceramide) and its related glycolipids in a variety of human cells, may play a role in the virulence of N. meningitidis by enabling the organism to evade host immune defenses. The expression of the antibody- reactive epitope in the LOS was influenced by growth conditions and the Lnnt epitope could be masked by a sialic acid which was identified as N- acetyl-neuraminic acid. We are currently investigating the linkage of the sialic acid to Lnnt in meningococcal LOS and its possible function.